This invention relates to compositions of propylene polymers suitable for high energy radiation treatment and to articles manufactured from such polymer compositions.
Propylene polymers, and specifically those of a substantial crystalline content have been applied to many new uses, which uses have required that the polymer be processed or treated in a particular manner. For example, propylene polymers generally have the contact clarity, heat distortion resistance and low chemical reactivity required by manufacturers of medical single-use items such as syringes, forceps, surgical clamps and various instrument trays, etc. used in the operating room. Obviously, before any such articles can be safely used, it is required that it be sterilized.
Ethylene oxide has the sterilizing effectiveness needed by these same manufacturers, and has been regarded in the past as generally safe. Recently, however, ethylene oxide has come under increasing scrutiny by the authorities, since it is believed to be a mutagen and possibly a carcinogen. Acceptable limits of residual ethylene oxide and its by-products have, therefore, been lowered. In addition, the cost of ethylene oxide is rising and the sterilizing costs (because of a 14-day holding requirement and the individual batch testing requirement) are becoming non-competitive.
High energy radiation, on the other hand, leaves no residue and has a dose-related sterilization rate which permits immediate release upon certification of the dose. However unlike ethylene oxide, radiation, especially gamma radiation, damages the polymer in that it causes either embrittlement or discoloration or both of these effects occur simultaneously. As disclosed in U.S. Pat. No. 3,537,967, the discoloration is attributable to the use of phenolic antioxidants in the compositions to prevent or minimize the radiation-induced degradation of the polymer. Even worse discoloration is obtained with additive systems containing phenolic antioxidants and a thiodipropionic ester synergist. The patent teaches that discoloration resistance upon high energy radiation is achieved by the incorporation into the polymer of the thiodipropionic ester as the sole stabilizer. However, these compositions have been found not to be entirely satisfactory, especially for use in the fabrication of syringes in that the strength retention (flexural strength) of the irradiated polymer is minimal, even at high levels of thiodipropionic ester incorporation and the sterilized articles become so embrittled that they are prone to breakage in use.
European Pat. No. 7736 teaches that if certain hindered amines are incorporated into a polyolefin, such as propylene homopolymer, there is obtained an improvement in resistance to discoloration which occurs as a result of gamma radiation. The patent discourages the use of phenolic antioxidants in the composition since they are likely to cause aggravated discoloration. However, the absence of an antioxidant will result in processing problems during molding, and sterilized articles manufactured from the composition will lack the required shelf life.
It is, therefore, an object of the present invention to provide novel compositions of matter of polymers of propylene which are resistant to discoloration and to degradation of physical properties upon high energy treatment.
It is a further object of this invention to provide shaped articles of polymers of propylene which can be used for medical or food packaging purposes, which shaped articles will be subjected to a sterilizing dose of high energy radiation.